The Origin and Evolution of the Oceans PDF

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3 The Origin and Evolution of the Oceans Daniele L. Pinti 3.1 Introduction Oceans play a key role in the evolution of life. The first organic molecules on Earth have been likely synthesized in aqueous solutions and primitive biota possibly survived near oceanic hydrothermal systems (Stetter 1998, Ho...

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3 The Origin and Evolution of the Oceans Daniele L. Pinti

3.1 Introduction Oceans play a key role in the evolution of life. The first organic molecules on Earth have been likely synthesized in aqueous solutions and primitive biota possibly survived near oceanic hydrothermal systems (Stetter 1998, Holm and Andersson 1998, Nisbet and Sleep 2001). The oceans shielded organic molecules from the massive UV radiation (Cleaves and Miller 1998) and protected living organisms from the heavy cometary and meteoritic bombardment of our planet (Sleep et al. 2001, Nisbet and Sleep 2001). Nonetheless, our knowledge of the origin and the evolution of the oceans is rather poor and a universally accepted model of formation for the terrestrial oceans is not yet available. The main reason is that the atmosphere–ocean system formed during the first 700Ma of the history of the Earth (Fig. 3.1). During this period, called “Hadean” (from “Hadeus”, the Greek god of Hell) and that extends from the Earth accretion to the end of the heavy meteoritic bombardment, 3.9Ga (Ga = billion years), the geological record has been mostly wiped out by the intense tectonic activity of the young Earth. The Hadean geological record is reduced to a handful of detrital zircons found in Western Australia (Wilde et al. 2001, Mojzsis et al. 2001). These zircons contain precious information on the presence of liquid water, very early in the history of the Earth, and their study is revolutionizing our geological view on the primitive Earth (Peck et al. 2001, Valley et al. 2002). Perhaps, liquid water occurred at the surface of the Earth 50 Ma after the end of the accretion. A few tens of millions of years later, the oceans may have reached the conditions of temperature, salinity and pH suitable for the survival of living organisms, probably extremophiles (Rothschild and Mancinelli 2001), and this well before the end of the heavy meteoritic bombardment of the Earth. Occasionally, the impact of large asteroids could have boiled the ocean and momentarily sterilized the Earth (Nisbet and Sleep 2001) In this chapter, I will summarize the current state of knowledge on the origin of the oceans on the basis of theoretical models and the few geochemical and isotopic records that the primitive oceans have left in Precambrian rocks.

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Fig. 3.1. Sketch showing the chronology of the first events that resulted in the formation of the oceans. Surface temperatures are indicative because the transition from uninhabitable hot to uncomfortably cold Hadean climates has been also suggested for the Hadean period (e.g. Sleep and Zahnle 2001). The first traces of life in apatite at 3.8 Ga are graphite particles of possible organic origin found in Isua metasediments (Mojzsis et al. 1996). Recently, van Zuilen et al. (2002) questioned this result, suggesting an inorganic origin for the carbon. The age of Acasta orthogneisses are from Bowring and Williams (1999)

3.2 The Origin of Water All theoretical models of formation of the oceans need a clear answer to a basic question: when and how water was delivered to our planet. However, the origin of water remains one of the most important subjects of debate and controversy in geosciences and astrophysics.

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Knowing the timing of water delivery to the Earth may allow a choice to be made among the possible scenarios of the ocean formation proposed until now: (1) an early delivery of water during the accretion of the Earth, which implies that the oceanic water inventory was available since the beginning (Dauphas et al. 2000, Morbidelli et al. 2000, Robert et al. 2000); or (2) a continuous delivery of water through eons, which implies expanding oceans (Frank 1986, Deming 1999). Currently, the most accepted hypothesis is the former, but the extraterrestrial carrier of the water (chondritic vs. cometary) and the precise moment of the delivery (during the planetary growth or at the end of accretion), are still matters of debate (Owen 1998, Delsemme 1999, Morbidelli et al. 2000, Dauphas et al. 2000, Dauphas 2003). I would like to briefly discuss the long-standing hypothesis of a constant delivery of water throughout the history of the Earth, which implies an expanding ocean, if we assume that there is not a return of water to the mantle. It is mainly based on the theory of Frank et al. (1986) who showed, through a series of measurements on spacecrafts, that the Earth is annually hit by a large number of small cometary bodies. Their calculations indicated that about 2.2– 8.5 × 1021 kg of water has reached the Earth since its formation, if the influx rate is assumed constant. This is equivalent to three times the mass of water in the present-day oceans (1.4 × 1021 kg). More recent data of Frank and Sigwarth (1997) collected by the ultraviolet imagery of the POLAR spacecraft confirmed this finding. The cometary hypothesis is not universally accepted. Harrison (1999) showed, using models of the variations of the continental freeboard, that the constant addition of large quantities of water (comparable to the present-day ocean volume) throughout the history of the Earth is an unlikely process. His conclusion is also supported by geological evidence of a deep water table at the surface of the Earth, since the Archean. The minimum water depth needed to form the 3.2Ga Ironstone Pods in the Barberton greenstone belt is 982m (de Ronde et al. 1997). Volcanic massive sulfides are also common in Archean terrains. Some of them are analogous to sulfide deposits produced at present-day midocean ridges. To produce such a deposit, the ocean-floor pressure should be higher than the critical pressure, which is equivalent to an ocean depth of about 3 km (Harrison, 1999). The universally accepted hypothesis is that the terrestrial water inventory (Table 3.1) was available soon after the Earth formation. A first hypothesis suggests that water and other volatiles degassed from the interior of the Earth, at the moment of its formation (Rubey 1951). The second hypothesis suggests that a few planetary embryos accreted by the Earth at the final stage of its formation carried the bulk of water presently on Earth (Morbidelli et al. 2000). These planetary embryos may have had a chondritic composition and be originally formed in the outer asteroid belt (as the Trojan-class asteroids) (Morbidelli et al. 2000). Several authors challenged this interpretation, proposing alternative carriers of water such as chondritic micrometeorites (Engrand et al. 1999, Maurette et al. 2000) or comets (Delsemme 1999).

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Table 3.1. Concentration of water and D/H ratios in terrestrial and extraterrestrial reservoirs Reservoirs

Mass or concentration of H2 O [kg]

(×10−6 ) D/H

δD [%0 SMOW]

Ref.

Whole Earth Primitive Earth Mantle Oceans Organic matter Metamorphic rocks Sedimentary rocks PSN Carbonaceous chondritesa Antarctic micrometeoritesa Cometsa

2.2 – 6.7 × 1021 − 5 – 50 × 1020 1.40 × 1021 1.36 × 1018 3.60 × 1019

149 – 153 128 – 136 143 – 149 155.7 135 – 145 140 – 146

−40 to −20 −180 to +130 −80 to −40 0 −130 to −70 −100 to −60

1 1, 2 3 3 3 3

2.32 × 1020

143 – 145

−80 to −70

− 6 – 22

21 ± 5 128 – 181

−865 −180 to +160

4 5–8

2–4

120 – 200

−229 to +285

9, 10

58 – 65

298 – 324

+900 to +1080

3, 4, 8

3

a

H2 O concentration in wt%. References: [1] Dauphas et al. 2000, [2] D´eloule et al. 1991; [3] L´ecuyer et al. 1998; [4] Robert 2001; [5] Boato 1954; [6] Robert and Epstein 1982; [7] Kerridge 1985; [8] Morbidelli et al. 2000; [9] Engrand et al. 1999; [10] Maurette et al. 2000.

In a pioneering work on the origin of the oceans, Rubey (1951) tested different hypotheses. First, he argued that water derived from the weathering of the continental crust, which is known to contain large amounts of water, mostly in the form of hydrated minerals. However, using a simple mass balance, Rubey showed that the amount of water contained in the silicate rocks is insufficient to deliver water to the oceans. The continental crust has a mass of 2.4 × 1022 kg and it contains about 1% of H2 O (Krauskopf and Bird 1995). Even assuming that all the continental crust has been weathered, only 10% of the terrestrial water inventory could have been delivered through this mechanism. Rubey considered another potential source of volatiles, i.e. volcanism. He observed that gas emissions from volcanoes are mainly composed of H2 O and CO2 with minor amount of sulfates, nitrogen and rare gases. This corresponds to the volatile composition of the atmosphere, oceans and sediments. He argued that the degassing of the volatiles trapped during the Earth accretion, via the volcanism, could explain the formation of the atmosphere and the oceans.

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The degassing of the Earth’s interior has been largely demonstrated by noble gas studies (e.g., All`egre et al. 1986, Ozima and Podosek 2001). During the first oceanographic cruises at Galapagos and EPR midocean ridges, at the end of the 1970s, an anomalous enrichment of the primordial isotope helium-3 was detected from deep water sampled at the axis of the ridge (Craig et al. 1975). The ratio between the primordial and the radiogenic isotope of helium, namely 3 He/4 He ratio, was higher than that expected in seawater, which should correspond to the atmospheric one. The degassing of primordial volatiles from the upper mantle was the cause of the observed enrichment of 3 He. The analyses of primordial noble gases in mantle rocks such as MORBs (midocean-ridge basalts), diamonds, xenoliths (e.g., Sarda et al. 1988, Staudacher et al. 1989, Ozima and Zashu 1988, Poreda and Farley 1992) and the discovery of extinct radionuclides, such as the Pu-I-Xe system (Butler et al. 1963, Staudacher and All`egre 1982, Marty 1989) has clearly demonstrated a catastrophic degassing that took place in the first 100Ma after the Earth accretion. The observations of noble gas having a clear “solar” isotopic composition, such as neon, was another piece of evidence supporting the degassing of primordial gases trapped during the formation of the Earth from the solar nebula (Honda et al. 1991). It has been often considered, by comparison with noble gases, that the major volatiles (N, C, O, H) had the same origin. The hypothesis of Rubey implies that the volatiles trapped during the formation of the Earth derive from the gas and dust of the protosolar nebula (PSN hereafter). However, the proximity of the Earth to the Sun implies high condensation temperatures that cannot allow incorporation of highly volatile elements such as N, C, O, H. Furthermore, the redox conditions that prevailed in this sector of the Solar System did not allow water to form, suggesting a source different from PSN for the oceans. This is confirmed by the isotopic signature of water and particularly by the isotopic ratio of hydrogen (D/H) measured in modern seawater (value of the standard mean ocean water, SMOW), which is equal to 155.7 × 10−6 (δDSMOW = 0 %0; Table 3.1). The D/H ratio is highly variable in the Solar System, but in a general way, it increases moving outward the Solar System due to a progressive enrichment of deuterium. This enrichment is produced by ion–molecule interactions in the interstellar medium (Robert et al. 2000). The D/H ratio in the PSN has been estimated to be 21 ± 5 × 10−6 using as reference the isotopic composition of helium in the Sun (Geiss and Gloecker 1998) and the isotopic composition of molecular hydrogen in the high atmosphere of the giant planets (Gautier and Owen 1983). The value of 21 ± 5 × 10−6 is more than seven times less than the D/H ratio measured in the oceans. Using appropriate models of the solar nebula, Drouart et al. (1999) showed that only planetesimals formed in the region of Jupiter–Saturn and in the outer asteroid belt could contain water with a D/H ratio similar to that of the oceans. Only two types of planetary bodies could be the source of this water: comets and hydrous carbonaceous chondrites (Morbidelli and Benest 2001).

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Comets are bodies composed of ices (mainly H2 O and in minor amounts CH3 OH, CO and CO2 ) together with particles of silicates, carbon and organic matter. The comets actually observed, such as Halley or Hale-Bopp, likely come from the Oort Cloud, a distant cloud of comet material located in the outer region of the Solar System, itself populated mostly by planetesimals that were originally in the Uranus–Neptune region and in the primordial Kuiper Belt. Among the most primitive meteorites, carbonaceous chondrites contain a large amount of water (up to 22 g of H2 O per 100g of rock; Kerridge 1985). Currently, these meteorites are rare and only 4% of the falls on Earth corresponds to carbonaceous chondrites (Dauphas and Marty 2001). However, hydrous carbonaceous chondrites seem to be a large reservoir of extraterrestrial material: the outer asteroid belt, located between Mars and Jupiter at 2 UA and that is the major source region of the meteorites arriving currently on Earth, seems to be dominated by carbonaceous chondrites; the lunar soil contains from 1 to 2% of such material; the Antarctic micrometeorites (AMM), that constitute the highest flux of extraterrestrial material actually reaching the Earth (40 000tons per year), are hydrous carbonaceous chondrites (Engrand et al. 1999, Maurette et al. 2000). To choose between these two potential candidates, we have a strong isotopic constraint, which is the ratio between deuterium and hydrogen (D/H ratio) in the water molecule. The values of the D/H ratios in the terrestrial and extraterrestrial reservoirs (PSN, AMM, carbonaceous chondrites, comets) are reported in Fig. 3.2 and Table 3.1. The water of the oceans has a D/H ratio of 155.7 × 10−6 , while the whole Earth has a D/H ratio of 149– 153 × 10−6 (Lecuyer et al. 1998). These values are close to those measured in the carbonaceous chondrites (128– 180 × 10−6 , average 149 ± 6 × 10−6 ; Boato 1954, Robert and Epstein 1982, Kerridge 1985, Robert 2001) and those measured in the Antarctic micrometeorites AMM (140– 200 × 10−6 , average 154 ± 16 × 10−6 , Engrand et al. 1999, Maurette et al. 2000). The D/H ratio in the water of comets has been measured only in Comet Halley (316 ± 34 × 10−6 , Eberhardt et al. 1995), in Comet Hyakutake (290 ± 100 × 10−6 , Bockel´ee-Morvan 1998) and in Comet Hale-Bopp (320 ± 120 × 10−6 , Meier et al. 1998). The obtained values are 10– 20 times larger than the D/H ratio of molecular hydrogen in the PSN and 2 – 3 times larger than the value found for modern seawater, suggesting that comets did not contribute significantly to the delivery of water to Earth. A simple mass and isotopic balance shows that the amount of water delivered by comets on Earth is around 10% of the total (Dauphas et al. 2000). This is in agreement with independent estimates, based on the mean collision probability with Earth of comets coming from the giant planet region calculated by Morbidelli et al. (2000). This calculation indicates that ∼ 5 × 10−5 MT (MT is the mass of the Earth = 6 × 1024 kg) of cometary material from the trans-Uranian region could have been accreted to Earth. Even assuming a 100% water composition of material and a 100% impact efficiency, this would imply the delivery of only 10 – 15% of the terrestrial water inventory.

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The defenders of a cometary origin of water observed that the comets in which we measure the D/H ratio are “long-period” comets, probably formed in the Uranus–Neptune region or in the trans-Uranian region (Delsemme 1999). Accepting the model of D/H enrichment in the PSN (Robert et al. 2000), it is understandable to find D/H ratios 10 times higher than that of the PSN in the outer regions of the Solar System. Owen and Bar-Nun (1995) and Delsemme (1999) suggested that comets delivering water on Earth were those formed in the Jupiter region. These comets would have formed at relatively high temperature (100K) and consequently they would exhibit a lower D/H ratio due to exchange with protosolar hydrogen. Morbidelli et al. (2000) showed that the lifetime of comets from this region is extremely short (estimated lifetime of 1.5 × 105 y), lowering the impact probability of these comets with an accreting Earth. They concluded that the contribution of cometesimals to the terrestrial water inventory was negligible.

Fig. 3.2. Frequency distribution of the D/H ratios measured in carbonaceous chondrites and Antarctic micrometeorites compared to values for the PSN, Earth oceans and comets. Data: D´eloule et al. 1991, Engrand et al. 1999, Maurette et al. 2000, Robert et al. 2000

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We can understand from these studies that the debate is far from settled. The accepted hypothesis is that 90% or more of the water has been delivered to Earth by small asteroids having a chondritic composition and coming from the outer asteroidal belt (Morbidelli et al. 2000). The timing of delivery is likely at the end of the accretion of the Earth (Dauphas 2003). This is suggested by the presence of anomalous amounts of siderophile elements in the mantle (Dauphas and Marty 2001). The siderophiles are elements (such as those of the platinum group) that should have migrated to the core during the Earth primary differentiation, together with Ni-Fe alloy. However, a non-negligible amount of these elements is still present in the mantle and this can only be explained by a late addition, after the core formation and the mantle separation. The core formation is estimated at 50 Ma after the condensation of the PSN (All`egre et al. 1995). The relative abundance of siderophiles in the mantle is very similar to that of the primitive meteorites, such as the hydrous carbonaceous chondrites. Mass-balance calculations suggest that a meteoric flux equivalent to 4.5 × 10−3 MT could explain the late delivery of siderophiles to the mantle (Dauphas and Marty 2001). The same carbonaceous chondrite-like bodies, containing from 6 to 22 wt% of water (Table 3.1), could have delivered 1.6− 6.0 × 1021 kg of water to Earth, from 1 to 4 times the mass of the present-day oceans (1.4 × 1021 kg).

3.3 Formation of the Oceans: the Geological Record If different views exist on the origin of water, there is a general consensus that the total oceanic water inventory was available soon after the end of the Earth accretion. The question is when did water condense on the surface to form stable oceans, habitable for life. The chronology of the ocean formation is mostly unknown and it depends on the physical parameters used for the different models of formation and the scarce geological records of the presence of liquid water at the surface of the Earth. The oldest marine sediments found so far are BIF (banded iron formation) in a layered body of amphibolite and ultramafic rocks, crosscut by a quartz-dioritic dyke having a U-Pb age of 3.865 ± 11 Ma, at Akilia, West Greenland (Nutman et al. 1997). Other traces of ancient marine sediments have been found at the Isua Supracrustal Belt, Southern West Greenland and consist of metamorphosed pelagic sediments, probably turbidites, dated at 3.7Ga (Rosing et al. 1996). New geological evidence of liquid water comes from the recent U-Pb dating of individual detrital zircons (ZrSiO4 ) from the Mt. Narryer quartzite and from Jack Hills metaconglomerate, Western Australia (Mojzsis et al. 2001, Wilde et al. 2001). Zircon is a common U-rich trace mineral in granitic rocks that preserves a detailed record of the magma genesis (Pidgeon and Wilde 1998). Furthermore, the radioactive parent nuclides 235,238 U decay into the stable daughter products 206,207 Pb. Both the parent nuclides and the daughter products can be preserved

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